During hyperbaric decompression, gas-supersaturation of tissues and blood can cause bubbles to form there with serious medical consequences. The growing use of dysbaric environments by man has made this problem of decompression sickness increasingly important. The objective of the present physiological study is to increase the understanding of the early etiology of decompression sickness. Essential information is lacking as to where and how the bubbles form, and the factors which cause the dramatic lowering of liquid cavitation stability in vivo are still to be established. Several basic aspects of bubble formation in living systems will be examined, using various unicellular and multicellular organisms, biological fluids and blood cells. We will attempt to determine for example (a) if gaseous nuclei commonly exist in organisms, (b) if spontaneous bubble nucleation can occur at gas-supersaturations that normally lead to in vivo bubbles, (c) the principles cause(s) of the apparent decrease in gas-supersaturation tolerance with increasing anatomical and morphological complexity, and (d) the conditions for intracellular bubble nucleation. The methods to be used include microscopic observations and cinemicrographic recordings of events during decompression while manipulating the gas saturations and the rates of compression and decompression. The results obtained will be used to formulate new approaches and to reevaluate existing concepts relating to formation of bubbles in vivo.